This invention relates to an inspection method using nuclear magnetic resonance (NMR). More in particular, it relates to an inspection method using NMR which can quickly measure a spatial distribution of static magnetic field homogeneity and can adjust quickly static magnetic field homogeneity when a heterogeneous object is placed inside an inspection apparatus using nuclear magnetic resonance (MRI: magnetic resonance imaging).
In the MRI, adjustment of static magnetic field homogeneity (shimming) is indispensable in order to obtain excellent images and spectra having high resolution. In such MRI's, determination of the distribution of inhomogeneity of the static magnetic field is indispensable, too, in order to adjust static magnetic field homogeneity on the basis of the distribution of inhomogeneity. As discussed in Journal of Magnetic Resonance, 77, pp. 40-52(1988), for example, a method which determines static magnetic field homogeneity from phase data of echo signals obtained by the inversion of gradient magnetic fields for a homogeneous sample (object) such as water has gained a wide application in the past as the method of effecting such a measurement. FIG. 2 of the accompanying drawings shows an example of the method of measuring such phase data. Next, this method will be explained with reference to FIG. 2.
First of all, a spin of the sample (object) is excited by a 90.degree. pulse and gradient magnetic fields Gx, Gy are then applied to change the phase of the spin. Subsequently, after a 180.degree. pulse is irradiated, a spin echo is generated by the inversion of a gradient magnetic field Gz. In this instance, the inversion timing of the gradient magnetic field Gz is adjusted so that the interval between the point of time at which the echo signal becomes maximal and the 180.degree. pulse is different by a very small time .epsilon. from the interval .tau. between the 90.degree. pulse and the 180.degree. pulse. When the time .tau. passes away from the irradiation of the 180.degree. pulse, the phase shift of the spin resulting from static magnetic field inhomogeneity completely cancels out with the phase shift resulting from E(x, y, z) between 90.degree. pulse and 180.degree. pulse. For this reason, the image S(x, y, z) obtained by this measurement method contains only the phase data resulting from static magnetic field inhomogeneity during the very small time .epsilon.. If a gyromagnetic ratio and the real and imaginary portions of the image S(x, y, z) are hereby represented by .gamma., Sr(x, y, z) and Si(x, y, z), respectively, static magnetic field inhomogeneity is given by the following equation: ##EQU1##
To adjust this static magnetic field homogeneity, it is customary to generate an additional magnetic field for correcting an inhomogeneous magnetic field by adjusting the values of currents flowing through multiple channels of a coil generally referred to as a "shim coil". To this end, it is necessary to delicately change the current value flowing through the multichannel coil for each channel so as to measure in advance shim characteristics from the change of the distribution of the static magnetic field, as described in Journal of Magnetic Resonance, 85, pp. 244-254 (198), for example.